Long Term Evolution (LTE) is the fourth generation mobile phone system being developed through the 3rd Generation Partnership Project (3GPP). Coordinated multi-point (CoMP) and Relay are advanced techniques introduced in 3GPP LTE in order to improve system performance, especially at cell edges. Multipoint communication here involves synchronized communication with multiple nodes in a network.
According to Qualcomm, 3GPP R1-084400, “Coordinated Multi-Point downlink transmission in LTE-Advanced”, Nov. 10-15, 2008, 3GPP TSG-RAN WG1 #55, the CoMP transmission techniques can be broadly categorized as joint processing, cooperative Beamforming, and cooperative silencing.
However, the applications of these CoMP techniques have more or less a requirement for coordinated multi-point synchronization. The coordinated multi-point synchronization have shown to be advantageous for coordinated multi-point processing, like macro-diversity reception see for instance Texas Instruments, 3GPP R1-093168, “Timing Advanced commands in support of UL COMP operation”, Aug. 24-28, 2009, 3GPP TSG-RAN WG1#58, Shenzhen, China and Texas Instruments, 3GPP R1-091293, “Enabling Coordinated Multi-point Reception”, Mar. 23-27, 2009, 3GPP TSG RAN WG1 #56bis, Seoul, Korea.
The requirements of the coordinated multi-point synchronization in a multi-point communication system can be coarsely classed to a loose synchronization and a tight synchronization. Loose synchronization can be seen as, User Equipment (UEs), normally cellular phones, communicating with a node having to adjust their transmission timing so that their signals arrive approximately simultaneously to the receiver, i.e. within a CP (cycle prefix) tolerance.
Loose synchronization can contribute to coordinated multi-point processing e.g. a macro combination processing on signals from the multiple nodes. At least the delay diversity gain can be obtained even if the signals from the multiple nodes are not strictly synchronized. However, if a tight synchronization is used, the signals from/to multiple cooperative nodes should arrive at a corresponding entity with which these nodes communicate within a small timing window, which may be the case in for instance cooperative beamforming. The tolerated receiving timing error of such signals for a beam forming system is much lower than a sampling duration (Ts) and can for example be a fraction of the sampling duration (Ts), e.g. 1/32 Ts, 1/4  Ts. A tight synchronization can further contribute to the obtaining of the coordinated antenna array gain through a joint transmission processing on the multi-point signals.
A major problem which occurs in a CoMP system is that the signal propagation delays from/to the mobile station, to/from different cooperating units, such as to and from different base stations, also denoted evolved Node B (eNB), are typically different.
Take an Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) system as a an example. If there is a serving base station and a mobile terminal then they can both obtain information about the propagation delay between them through timing advance and measurement reporting procedures. A first serving base station can in this way obtain the propagation delay between itself and the mobile station, and when the mobile station is handed over to another cell covered by a second base station, this new serving base station can obtain the propagation delay between itself and the mobile station. During this procedure, the mobile station can thus obtain information about the propagation delays in relation to both base stations. However, the first and second base station can not obtain each other's propagation delay to the mobile station. This may limit some usages of a CoMP system.
As mentioned above propagation delay can be obtained through using timing advance. Two network-initialized synchronization methods via explicit TA command signal transmission are proposed in Texas Instruments, 3GPP R1-093168, “Timing Advanced commands in support of UL COMP operation”, Aug. 24-28, 2009, 3GPP TSG-RAN WG1#58, Shenzhen, China and Texas Instruments, 3GPP R1-091293, “Enabling Coordinated Multi-point Reception”, Mar. 23-27, 2009, 3GPP TSG RAN WG1 #56bis, Seoul, Korea. The first document describes using a primary cooperating unit to collect all the propagation delays between a mobile station and all the cooperating units (base stations) and then deliver a TA command to the mobile station. The second document describes how the above mentioned responsibilities of the primary cooperating unit are taken by a central location. These network synchronization methods can help the serving base station to know the propagation delay not only between itself and any mobile station but also between the non-serving base stations or other coordinated base stations and the mobile station. However, these methods use frequent S1/X2 communications, and more importantly, the propagation delay values obtained may not be precise enough. The granularity of a TA command is for example 12 Ts in E-UTRAN system, see 3GPP TS36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation. This restricts the use of such TA commands to a loose synchronization system. It can therefore not be used in a tight synchronization system.
There is therefore a need for an improvement on this situation in this field of technology.